Structure and Function
Neurotoxins, such as those obtained from Clostridium botulinum and Clostridium tetani, are highly potent and specific poisons of neural cells. Both the single known tetanus toxin and the multiplicity of known botulinum toxins comprise, in their activated forms, two peptide acid chains coupled through a disulfide link: a light chain (LC) of about 50 KDa and a heavy chain (HC) of about 100 KDa. The toxins are synthesized in vivo as single chains, which are not toxic. However, the toxin becomes active when the single chain is nicked in a post-translational modification to form the separate LC and HC (linked by S-S).
The tetanus and botulinum toxins have lethal doses in humans of between 0.1 ng and 1 ng per kilogram of body weight. They function by inhibiting neurotransmitter release in affected neurons. The tetanus neurotoxin (TeNT) acts mainly in the central nervous system, while botulinum neurotoxin (BoNT) acts at the neuromuscular junction and other cholinergic synapses in the peripheral nervous system. Both types act by inhibiting acetylcholine release from the axon of the affected neuron into the synapse, resulting in paralysis. The effect of intoxication on the affected neuron is long-lasting and until recently has been thought to be irreversible.
Only one form of tetanus neurotoxin is known; seven different immunologically distinct forms of botulinum neurotoxins termed BoNT/A through BoNT/G are known. While all of these types are produced by isolates of C. botulinum, two other species, C. baratii and C. butyricum also produce toxins similar to /F and /E, respectively.
Regardless of type, the molecular mechanism of intoxication appears to be similar. First, the toxin binds to the presynaptic membrane of the target neuron through a specific interaction between the heavy chains (HC) and a cell surface receptor; the receptor is thought to be different for each type of botulinum toxin and for TeNT. The carboxyl terminus of the HC appears to be important for targeting of the toxin to the cell surface.
In the second step, the toxin crosses the plasma membrane of the poisoned cell, is engulfed by the cell through receptor-mediated endocytosis, and an endosome containing the toxin is formed. The toxin then escapes the endosome into the cytoplasm of the cell. The escape is thought to be mediated by a conformational change brought about by the acidic environment within the endosome which is effected by a proton pump that decreases intraendosomal pH. At a pH of about 5.5 or lower, the sequence at the amino terminus of the heavy chain triggers this conformational change. The conformation shift exposes hydrophobic residues which permits the toxin to embed itself in the endosome membrane and then translocate into the cytosol.
Once in the cytosol, reduction of the disulfide bond joining the HC and LC takes place. The entire toxic activity of botulinum and tetanus toxins is contained in the LC; which is a zinc (Zn++) endopeptidase that selectively cleaves “SNARE” proteins essential for recognition and docking of neurotransmitter-containing vesicles with the cytoplasmic surface of the plasma membrane, and fusion of the vesicles with the plasma membrane.
The “SNARE” proteins are of several forms which have differential responses to the various forms of toxin. TeNT, BoNT/B BoNT/D, BoNT/F, and BoNT/G cause degradation of synaptobrevin (also called vesicle-associated membrane protein (VAMP)), a synaptosomal membrane protein. Most of the cytosolic domain of VAMP extending from the surface of the synaptic vesicle is removed as a result of any one of these cleavage events. Each toxin (except TeNT and BoNT/B) specifically cleaves a different bond.
BoNT/A and /E selectively cleave the plasma membrane-associated protein SNAP-25; this protein is predominantly bound to and present on the cytosolic surface of the plasma membrane. BoNT/C cleaves syntaxin, an integral protein having most of its mass exposed to the cytosol. Syntaxin interacts with the calcium channels at presynaptic terminal active zones.
Both TeNT and BoNT are taken up at the neuromuscular junction. BoNT remains within peripheral neurons, and blocks release of the neurotransmitter acetylcholine from these cells. TeNT enters vesicles that move in a retrograde manner along the axon to the soma, and is discharged into the intersynaptic space between motor neurons and the inhibitory neurons of the spinal cord. At this point, TeNT binds receptors of the inhibitory neurons, is again internalized, and the light chain enters the cytosol to block the release of the inhibitory neurotransmitters 4-aminobutyric acid (GABA) and glycine from these cells.
Pharmaceutical Applications
Dilute preparations of BoNT have been used since 1981 as therapeutic agents in the treatment of patients having various spastic conditions, including strabismus (misalignment of the eye), bephlarospasm (involuntary eyelid closure) and hemifacial spasm. See e.g., Borodic, et al., Pharmacology and Histology of the Therapeutic Application of Botulinum Toxin in Therapy with Botulinum Toxin 119-157 (Jankovic J. & Hallett, eds. 1994), hereby incorporated by reference herein. The toxin preparations are delivered specifically and locally to the site of the neurons to be effected. BoNT/A is the most potent of the BoNT's, and the best characterized. Intramuscular injection of dilute preparations of BoNT/A has also been used effectively to treat spastic conditions due to brain injury, spinal cord injury, stroke, multiple sclerosis and cerebral palsy. The extent of paralysis depends on both the dose and volume delivered to the target site.
Clearly, it is desirable to confine the activity of the administered toxin to the target site. A number of strategies have been adopted, including, besides direct injection, implantation of a capsule pump or administration of a slow release gel. However, the success of such attempts has been far from complete. Because of the diffusion of the toxin from the site of administration, systemic problems, such as difficulty in swallowing have occurred. The reality of these undesired effects has limited the level of dosage which can be administered. For example, subjects needing treatment in both arms or both legs generally cannot be administered the toxin in both affected limbs simultaneously due to the side effects. It would thus be desirable to provide a form of the toxins which inherently acts specifically at its target site. The present invention provides such modified forms.